Storm-Time Ionospheric Response during the 11–12 November 2025 Geomagnetic Storm: Evidence of Anomalous TEC Double-Peak and Noon-Time Bite-out

This study investigates the ionospheric response to the geomagnetic storm of 11–12 November 2025, characterised by a minimum Dst index of −217 nT, using Global Ionospheric Maps of Total Electron Content (GIM-TEC) together with TEC measurements from low-latitude International GNSS Service (IGS) stations. The analysis provides a comprehensive characterisation of the spatial and temporal evolution of storm-time ionospheric dynamics across the equatorial and low-latitude regions. Quiet-time reference levels were computed as the mean TEC of the two quiet days immediately preceding storm commencement, and stations were selected to span the Equatorial Ionisation Anomaly (EIA) crest and trough across the Indian longitude sector, with geomagnetic latitudes noted for context. An anomalous morning-time double-peak TEC structure is identified around 10:00 LST (IST) on 12–13 November 2025, through visual inspection of its deviation from the conventional diurnal TEC profile, with the deviation magnitude calculated relative to the quiet-day reference, showing TEC enhancements of 10–20 TECu relative to quiet-time reference levels, the magnitude varying with station location. This unusual departure from the typical single-peak diurnal TEC pattern near the EIA is attributed to enhanced Equatorial Electrojet (EEJ) activity that drives charge accumulation near the dawn terminator, thereby perturbing the background E×B drift and plasma fountain dynamics. In addition, a distinct noon-time TEC bite-out, with a decay of nearly 5–10 TECu, is reported at equatorial stations Hyderabad, IISc Bengaluru, and Port Blair, similarly attributed to enhanced EEJ activity, which removed and drifted the accumulated charge toward the dusk terminator, consistent with a reversal or weakening of the equatorial vertical drift during disturbed conditions. The observed TEC variations closely track the main, recovery, and initial/substorm-associated phases of the storm (defined using Dst/SYM-H onset, minimum, and recovery timing), reflecting the time-varying interplay between magnetospheric energy input and low-latitude electrodynamic response.  These findings underscore the pronounced sensitivity of the low-latitude and equatorial ionosphere to extreme space-weather forcing and highlight the value of coordinated multi-instrument, multi-station observations in resolving the complex electrodynamic.